1. Field of the Invention
2. Background Art
The dominant commercial process for manufacturing flat glass is the float process. In the float process, a stream of molten glass is discharged from a melting furnace into a furnace (or bath) that contains a liquid metal medium. Typically, the metal is tin. The atmosphere in the tin bath is controlled to prevent oxidation of the tin. The molten glass floats and spreads out on the liquid tin in the form of a flat, continuous ribbon. The ribbon of glass is conveyed into an annealing lehr or cooling tunnel, where it is cooled at a controlled rate to ambient temperature. The cooled glass has a flat, smooth surface that requires a minimum of further finishing by processes such as grinding and polishing.
The float process has generally been assumed to be the best method for fabricating large-area display panels, such as plasma display panels. Glasses for plasma display panels have several requirements. One such requirement is that the glass should have a high strain point. In general, a strain point of at least 580° C. is needed to prevent panel distortion during display fabrication. However, it is very difficult to form glasses having high strain points, or glasses whose forming temperatures are very high, in an enclosure containing molten tin. This is because tin has high vapor pressures at temperatures in excess of 1050 to 1100° C. At these high vapor pressures, the molten tin will vaporize and then subsequently condense in colder parts of the float furnace. The condensation may be sufficiently high to create what is referred to as “tin rain”—a situation that should be clearly avoided.
Another method for forming flat glass is known as the fusion draw process, a process developed by Corning Incorporated. The fusion draw process is a down draw process, also referred to as the overflow process. In the fusion draw process, a glass-forming melt flows into a refractory trough and then overflows in a controlled manner from either side of the trough. A key advantage of this process is that the surface of the sheet glass, which is ultimately formed, does not come in contact with any refractory material or other forming equipment. Another benefit of the process is that it yields a very flat and uniformly thick sheet of glass. As a result, no secondary processing is needed to obtain a smooth, flat, and uniform sheet of glass for display applications. The fusion draw process requires glasses exhibiting a relatively high viscosity at the liquidus temperature. Typically, it is desirable to form the glass at viscosities in the range of 105 to 106 poise to obtain optimum flatness and uniform thickness.
A brief description of both the fusion draw and float processes are given in a manuscript entitled “Glass” by D. C. Boyd and D. A. Thompson, Encyclopedia of Chemical Technology, Vol. 11, Third Edition, pp. 807–880 (see pages 860–863). The fusion draw process is also described in U.S. Pat. Nos. 3,338,696 and 3,682,609, both issued to Dockerty.